Nature-Inspired Adhesion Technology: Geckos

Adhesion Technology Inspired by Nature: Geckos The traces of adhesion technologies can be found deep in history. With the help of biologically-derived resins, hunting tools were made and developed, daily life was documented and simple treatment methods were applied. In joining through adhesion, tree resins and mussels (Figure 1); in the field of surface technology, the reptile family known as geckos has pioneered important research. [caption id="attachment_135913" align="aligncenter"] Figure 1: Adhesion Capability in Mussels (1)[/caption] Many animals such as insects, spiders and lizards have binding pads on their bodies, feet and legs, which possess the ability to adhere to and detach from various surfaces. For this reason, they move easily even on vertical walls and ceilings (2). Geckos are among the heaviest in this group of animals and, among climbing animals with intelligent adhesion capability, possess the most developed and complex foot structure. These organisms have approximately three billion nano-scale spatulae extending from approximately three million micro-scale hairs (setae) at a frequency of 14,000 per mm² on their feet (3). The setae found on gecko feet create numerous weak attractions between molecules on two surfaces, enabling suspension. Creating the bonds that each hair makes with the surface is, unlike any adhesive or tape, easy and rapid to break. For this reason, we can say that the natural properties possessed by this organism have inspired surface technologies. Scientists recreated gecko-like adhesion using silicone, plastic, carbon nanotubes and other materials, but encountered a scaling problem. In theory, adhesion strength is quite high—approximately 3-6 million setae could lift approximately 130 kilograms of mass. However, in reality a gecko can only carry a 2-kilogram load with its front feet. According to Stanford University researchers, the scaling problem stems from the fact that loads are not distributed evenly across broad adhesive areas. For example, on gecko toe pads, only some of the setae are in close contact with the surface, and those not in contact do not share the load equally among themselves. Researchers suggest that the reason for this may be that gecko skin, like a rubber band, becomes stiffer when more force is applied to it. When a lizard climbs a wall, the setae are stretched unevenly and some experience more force than others. For this reason, while some setae maximize their stickiness, others are underutilized or do not adhere to the surface at all. Therefore, researchers focused on distributing forces more evenly and developed an adhesion system based on the principle observed in geckos. Scientists began working with silicon micro-wedges that mimic the reptile's setae to create artificial gecko adhesives. They combined these into 24 scale-sized square tiles, each containing hundreds of thousands of micro-slits. The tiles were then attached to an octagonal plate by springs connected with tendon-like strings. Unlike gecko skin, the springs apply the same force to the tiles after being stretched beyond a certain threshold, distributing loads evenly among the tiles. This development work enabled the patch mounted on the surface to offer similar adhesion strength across dimensions from one square millimeter to the human hand. Even if a single tile were peeled off, its carried weight would be transferred to the tiles with the lightest loads that do not exceed the springs' threshold, keeping the overall system adhesive. The 7 parameters below associate polypropylene-based artificial gecko adhesives with the natural adhesion properties of this organism, which they recognize as their source of inspiration: 1. Directional Adhesion: Setae do not adhere by pressing against the surface; instead, a shear behavior parallel to the surface is required for the fibers to hold against the substrate. 2. High Preload/Shear Ratio: A preload of less than 0.1 Newton is sufficient to bind the fibers together, and after the preload is removed, each can withstand a 4-Newton shear load. 3. Low Separation Force: The polypropylene-based gecko adhesive is directional and exhibits adhesion parallel to the surface. However, it can be easily removed with a force of less than 0.001 Newton. 4. No Agglomeration: Polypropylene microfibers do not stick to each other and do not clump even during long loading/unloading cycles. 5. Non-Adhesive in Static State: Polypropylene, the polymer used in the synthetic adhesive, is nearly as hard as the gecko's keratin structure, and the patch surface is not adhesive in a static state. 6. Topography Independence: Can adhere to difficult surfaces. 7. Self-Cleaning: Research is ongoing. Stanford University researchers developed a robotic system called FarmHand (Farm Hand) in the Biomimetics and Dexterous Manipulation Laboratory. Materials successfully transported in tests include raw eggs, grape clusters, plates, liquid containers, basketballs and grinding motors. While using gecko adhesive in this multi-fingered, anthropomorphic gripper is challenging, it requires special attention to the tendons controlling FarmHand's fingers and the design of the finger pads beneath the adhesive. Like gecko toe pads, gecko adhesive provides a strong grip through microscopic fins. In robotic design, while these fins are in full contact with the surface, they adhere through Van der Waals force—a weak intermolecular force arising from subtle differences in the positions of electrons outside molecules. As a result, the adhesives possess strong gripping ability and the actual force required to achieve this is quite minimal. Additionally, they leave no residue and do not feel sticky. In the early 2000s when work in this field began and intensified, it was assumed that the bonding mechanism of setae on gecko feet was Van der Waals force, and in artificial applications, examples such as FarmHand and StickyBot (Figure 6) mentioned above heavily based this system. In 2014, Canadian researchers turned to the theory that the phenomenon enabling geckos to walk on surfaces thanks to the unique fibrillar nature of their foot pads is contact electrification (CE). In addition to adhesion force, by measuring electrical charges occurring between gecko setae and the surface, they conducted measurements confirming that the selective adhesion property on the surface operates through an electrical double layer (EDL) mechanism rather than capillary Van der Waals force at the contact interface. According to this, in the neutral state (Figure 7-a), gecko foot setae are placed on a polymer thin film spread over a copper sheet. When the foot comes into contact with the polymer film (Figure 7-b), electrical charges separate between the setae and the thin film. The EDL formed at the contact interface induces certain electrical charges on the grounded copper plate in the back, measurable with an electrometer. Electrical charges separated by contact (Figure 7-c) penetrate first into the polymer thin film (di), then into the seta depth (dg). TEFLON AF (amorphous fluoropolymer) has a weaker ability to create both Van der Waals and capillary forces compared to PDMS (polydimethyl siloxane), confirming once again the definitive contribution of CE-driven electrostatic interactions to gecko adhesion through the development of enhanced adhesion forces observed by TEFLON AF (8). References * https://www.shutterstock.com/image-photo/madagascar-day-gecko-phelsuma-madagascariensis-140581852 (1) Petrone, L., Kumar, A., Sutanto, C. et al. Mussel adhesion is dictated by time-regulated secretion and molecular conformation of mussel adhesive proteins. Nat Commun 6, 8737 (2015). https://doi.org/10.1038/ncomms9737 (2) https://www.science.org/content/article/gecko-inspired-adhesives-allow-people-climb-walls (3) Bhushan B. (2016) Gecko Effect. In: Bhushan B. (eds) Encyclopedia of Nanotechnology. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-9780-1_378 (4) https://plastics-themag.com/With-this-polymer-we-will-soon-be-able-to-make-geckos-feet-in-the-kitchen (5) https://en.wikipedia.org/wiki/Synthetic_setae (6) Kellar Autumn, Anne M. Peattie, Mechanisms of Adhesion in Geckos, Integrative and Comparative Biology, Volume 42, Issue 6, December 2002, Pages 1081–1090, https://doi.org/10.1093/icb/42.6.1081 (7) https://news.mit.edu/2009/stickybot-092509 (8) Izadi H, Stewart KME, Penlidis A. 2014 Role of contact electrification and electrostatic interactions in gecko adhesion. J. R. Soc. Interface 11 : 20140371. http://dx.doi.org/10.1098/rsif.2014.0371  

A. Tuğçe Onur Materials and Process Specialist Engineer Adhesive Engineer, EAE Turkish Aerospace Industries Inc.